Patricia Cohen explains how the famous portrait of Mary Todd Lincoln, which has hung in the Lincoln Presdential Library and Museum in Springfield, Illinois, for decades, has been determined to be not of Mrs. Lincoln but of an unknown woman. Barry Bauman, the conservator who pieced together the history of the altered portrait, says the man behind it and its story is Lew Bloom, who sold the entire package to Lincoln's graddaughter. The Lincoln Library will announce the findings on April 26th, the anniversary of the death of John Wilkes Booth.